Magnetic media conventionally comprise a magnetic coating on a non-magnetic substrate (support). The magnetic coating, which is generally applied as a suspension in an organic solvent and is subsequently dried, is basically made of fine magnetic pigment particles, such as iron oxide, dispersed in a polymeric resin binder, but may also contain lubricants and other additives. For many applications, such as video tape, computer tape, audio tape, floppy disks and rigid disks, the magnetic properties of the magnetic coating must be optimized in order to take advantage of the increased sophistication and capabilities of modern recording and computer hardware. Thus, for example, squareness ratio (SR) should be as high as possible, coercivity (Hc) should be high and switching field distribution (SFD) should be kept low. These properties are readily calculated from a magnetization curve (B- H curve) as illustrated, for example, in U.S. Pat. No. 4,438,156. Squareness ratio is equal to the quotient of retained magnetic flux divided by maximum magnetic flux and high values indicate greater retention of information stored in the magnetic media. Coercivity is a measure of the difficulty of erasing a recorded signal and high values result in improved "protection" of stored information. Switching field distribution is a measure of the variation in particle coercivity in a magnetic medium. A small SFD gives a well defined recording zone and increased output at short wavelengths. Of these variables, the squareness ratio is most important; it represents the effectiveness of the dispersion of magnetic particles, and high values result in increased long wavelength output of the magnetic medium.
Superior magnetic properties can, however, only be attained when the magnetic pigment is well dispersed in the medium, such that the individual magnetic particles do not interfere with each other. Unfortunately, the magnetic pigments, which are of microscopic dimensions, are difficult to disperse and often tend to agglomerate in the magnetic coating compositions. This difficulty has been resolved in the art to some degree by including a dispersant in the magnetic coating composition.
Early formulations employed small quantities of the natural product lecithin, or a phosphate ester, as dispersant. Use of a phosphate ester dispersant to achieve good dispersion, improved durability and reduced discontinuities is claimed in U.S. Pat. No. 4,419,257 to Frew et al. Therein, the phosphate ester is combined with a solvent system, which includes a dibasic ester, and said solvent system has specific Hansen three-dimensional solubility parameter values. Such dispersants do improve the dispersion quality in a magnetic coating composition, but they can not chemically bind to the pigment particles, and so are free to migrate within the magnetic coating composition, even when the latter is dried onto a substrate to form the magnetic medium. This free dispersant tends to plasticize (i.e., soften) the polymeric binder as well as migrate to the surface of a finished magnetic medium where it can, for example, mix with lubricant and thereby adversely affect frictional properties. Once at the surface of a magnetic medium, the dispersant can potentially oxidize, pick up debris or deposit on recording heads. These undesirable effects often become more pronounced as the amount of dispersant which is added to the magnetic coating composition is increased. Such an increase of dispersant level is generally necessary when high surface area or metal pigments are employed. Furthermore, the plasticization of the binder and ability of the dispersant to migrate away from the magnetic particles, even when the magnetic coating composition has dried, may permit some particle re-agglomeration which, in turn, leads to inferior magnetic properties with time. Because of such disadvantages, it is desirable to reduce the amount of phosphate ester dispersant in magnetic coating compositions.
Organosilanes having hydrolyzable groups have been employed in the art to improve dispersion. These materials are believed to form physiochemical bonds with reactive groups on the surface of the magnetic pigment. Moreover, silane coupling agents, which also contain functionality capable of reacting with the binder resin of a magnetic coating composition, may be employed. Thus, for example, Schonafinger et al., in U.S. Pat. No. 4,271,234, disclose the treatment of iron oxide pigment with various silanes including alkyltrimethoxysilane, vinyltrimethoxysilane, gamma-glycidyloxypropyltrimethoxysilane, gamma-aminopropyltriethoxysilane and methacryloxyethyltrimethoxysilane. When this treated iron oxide is formulated into a magnetic coating, improved dispersion of the pigment, as well as increased durability of the magnetic coating, is reported.
Chlorosilane and alkoxysilane coupling agents are taught by Yamada et al., in U.S. Pat. No. 4,076,890, to modify a magnetic coating mixture. In this case, a large number of silanes is disclosed, and incorporation of the silane into the composition may be by way of treating the magnetic pigment or by direct addition to said composition. The resulting magnetic media are claimed to be abrasion resistant and improved with respect to adhesion between magnetizable layer and support substrate, thereby exhibiting reduced powder dusting from tape edges.
The reaction product of a phosphoric ester with an polyisocyanate compound having at least two isocyanate groups, or an isocyanate compound having a hydrolyzable alkoxysilane, is disclosed by Takeuchi et al. in U.S. Pat. No. 4,501,795. When this reaction product is employed as a dispersant in a magnetic coating composition, good dispersibility of the magnetic powder and excellent durability of the magnetic layer (coating) are said to result.
Kimball, in copending application, Ser. No. 874,751, filed on June 16, 1986 and assigned to the assignee of this invention, discloses a dispersant for magnetic media which is the reaction product of a phosphate ester and certain organofunctional alkoxysilanes. The dispersant is said to be able to form chemical attachments with reactive groups on the magnetic particles and impart improved magnetic properties when compared with either the silanes alone or with the reaction products of phosphate ester and isocyanate-functional alkoxysilanes.